With two months until mid-term congressional elections, President Barack Obama's Democrats face increasingly long odds of maintaining control of the US Senate.
Republicans must gain six seats to take over, and many analysts say that goal is within reach.
Far more Democrats are up for re-election than Republicans this cycle, including four struggling to keep their seats in crucial swing states that Obama lost when he was re-elected in 2012.
But Republicans are hardly united in their tactics, as evidenced by pressure from core conservatives threatening a government shutdown over immigration policy -- tactics experts warn could backfire.
Nevertheless, the wind is at Republicans' backs in this mid-term election, which traditionally favors the opposition party in year six of a presidency, as this is.
Republicans are predicted to maintain control of the House of Representatives, where all 435 seats are being contested. Democrats are struggling to hang on in the 100-member Senate, where 36 seats are up for grabs.
Below is a list of key Senate races in November's election and issues to watch as the parties battle for congressional supremacy.
The race in this remote, independent-minded state features mud-slinging and vitriol, with freshman incumbent Mark Begich facing a strong challenge from Republican Dan Sullivan, appointed as attorney general in 2009 by then-governor Sarah Palin. Democrats accuse Sullivan of carpetbagging into Alaska. Each candidate has run controversial attack ads that were eventually pulled.
With Obama's approval in the cellar here, Begich seeks to stand as his own man, declining to invite the president on the campaign trail.
Incumbent Senator Mark Pryor is vulnerable, no doubt. He and Republican Tom Cotton, an Iraq and Afghanistan war veteran and popular young conservative congressman, are in a dead heat, according to Friday polls. But Pryor is using a card in this southern state played by few swing-state Democrats: "Obamacare."
Pryor highlighted his vote for Obama's controversial health care reform law in a campaign ad, and Cotton, who has advocated repealing the law, has only offered a muted response. Pryor was called out for fear-mongering when he accused Cotton of voting "against preparing America for pandemics like Ebola."
In the land of big oil, Senator Mary Landrieu is bracing for the political battle of her life against Republican Bill Cassidy, who is pegging her to unpopular Obama.
Like Begich, Landrieu has publicly criticized the White House for not authorizing construction of the Keystone XL oil pipeline from Canada to the Gulf Coast. And she is pressing her own initiatives for expanding offshore oil exploration.
Another state lost by Obama in 2012, North Carolina has experienced a conservative wave in recent years.
Voters have amended the state constitution to ban gay marriage, and lawmakers led by House speaker and Republican Senate challenger Thom Tillis approved ultra-conservative budget policies.
Incumbent Senator Kay Hagan has struck a centrist tone. She hits Obama on issues like veterans' care, but blasts Tillis for shortchanging education. Hagan boasts that a newspaper called her "not too far left, not too far right -- just like North Carolina."
Other vulnerable Senate Democrats include: Colorado's Mark Udall, whose approval ratings have slipped along with Obama's in a volatile swing state; retiring Tom Harkin from the breadbasket of Iowa, where the race for his seat is considered a toss-up; another retiring senator, Carl Levin of Michigan; and New Hampshire incumbent Jeanne Shaheen, who seeks to fend off former senator Scott Brown.
Red states Georgia, Kansas and Kentucky are weak spots in the Republican battle plan and could dash their hopes to retake the Senate, although polls there show Republicans hold slight edges.
Kentucky's race, pitting entrenched Senate Majority Leader Mitch McConnell, 72, against Alison Lundergan Grimes, a political rookie half his age, is on track to be the most expensive in Senate history.
Georgia features a tight race between Democrat Michelle Nunn, who seeks to follow in the footsteps of her senator father who famously claimed the ideological middle ground, and Republican David Perdue, a former Reebok chief executive.
And Kansas incumbent Pat Roberts, 78 and a symbol of the Senate's old guard, is under threat from the meteoric rise of independent Greg Orman, who is riding an anti-incumbency wave.
Statistical models clearly favor Republicans. Political blog FiveThirtyEight gives the party a 63 percent chance of winning Senate control in November; The New York Times, 62 percent; Huffington Post, 59 percent and The Washington Post, 53 percent.